How to make 3d text values larger in 3d view?
The font size for the dimensions is the same size as the text in panels throughout Blender (ie, the fields in the right-hand properties panel in your screenshot. You can adjust the overall font size by adjusting the DPI setting in the System section of the User Preferences :



Note that adjusting the DPI will affect the whole Blender user interface, not just the labels in the 3d View.